(I use both at times, nothing beats the cimplicity of the "Color to transparency option in
You must have intended to put [GIMP] at the end of that sentence, because the Color to Clear in GIMP is quick, easy, and reliable. At first, I believed that I would rarely use this function but I find many ways to Color to Clear in manipulating an image.
I use GIMP over 90% of the time, but Inkscape and Blender have some functions that are hard to find in no cost software. These software allow professional quality results in almost all situations despite being no cost. GIMP developers are sometimes the first anywhere to introduce a new tech function in GIMP before any others. They are very good at improving and expanding GIMP, so it will be in the picture as a competitive image manipulation software for a long time.
WordPress is another worth mentioning, though only a few functions would be commonly used for game development needs, however web games obviously could get more use from it. It is not directly for image manipulation, but you can make and change things in a browser window which can later be captured by screenshot and imported into GIMP or other ones.
Screen capture/screenshot is can be a major area contributing to game dev 2D work, so get a good software or two that has good screenshot ability such as FRAPS, GIMP, and so on.
When you get more advanced, then you may sometimes use video editing software to display and alter video images which can be used in part or the whole video frame for your game dev images. Video editing software can have some amazing effects! For example, if you had a WW2 game and wanted a WW2 theme splashscreen, then you could use a video editing software to get the right resolution and other tasks to get it just right and screenshot it to be imported into GIMP or other software. I have done this on occasion.